" Around the age of two, children realize that they’re separate from their mothers. In order to try out their muscles as individuals, they begin to disagree with those around them by saying no (hence the “terrible twos”). Toddlers who successfully detach from their mothers are able to say, in effect, “No, I will not eat what you want, put my boots on, or do what you say. I am a separate person.” This stage helps children learn the concept of “mine,” but it’s also part of learning to assert themselves. "

― Catherine Gildiner , Good Morning, Monster: A Therapist Shares Five Heroic Stories of Emotional Recovery